Stents are widely used for maintaining an open lumen in a patient's body. For example, stents may be used to maintain patency of a coronary artery, carotid artery, cerebral artery, popliteal artery, iliac artery, femoral artery, tibial artery, renal artery, other blood vessels including veins, or other body lumens such as the ureter, urethra, bronchus, esophagus, tear duct, fallopian tube, nasal cavity, or other passage.
Stents are commonly metallic tubular structures made from stainless steel, Nitinol, Elgiloy, cobalt chrome alloys, tantalum, and other metals, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, or metallic, and may elute over time substances such as drugs. Non-bioabsorbable stents may also release drugs over time. In certain designs, stents are open-celled or closed-celled cylindrical structures. Stents are passed through a body lumen in a collapsed state. At the point of an obstruction or other deployment site in the body lumen, the stent is expanded to an expanded diameter to support the luminal wall and maintain an open lumen at the deployment site.
One type of stent is often referred to as a “balloon expandable” stent. Stent delivery systems for balloon expandable stents are typically comprised of an inflatable balloon mounted on a multi lumen tube. The stent delivery system with stent crimped thereon can be advanced to a treatment site, often over a guidewire, and the balloon inflated to expand and deploy the stent.
In the case of a shape memory stent, the stent is pre-programmed to remember a reduced diameter or shape at one temperature and an expanded diameter or shape at a higher temperature. The stent is compressed onto the distal end of a stent delivery system at a cold temperature and the stent delivery system with stent crimped thereon can be advanced to a treatment site, often over a guidewire, while being maintained below a phase change temperature of the stent. At the deployment site, the stent is warmed or allowed to warm to a higher temperature at which a phase change of the shape memory material results in expansion of the stent from a collapsed state to an expanded state.
Other stents are so-called “self expanding” stents and do not use balloons to cause the expansion of the stent. An example of a self-expanding stent is a tube (e.g., a coil tube, a mesh tube, or a tube comprised of formed wire with or without welded wire junctions) made of an elastically deformable material (e.g., a superelastic material such a nitinol). Open or closed cell stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods. A very popular type of self expanding stent is made from superelastic nitinol, for example, the EverFlex stent made by ev3, Inc. of Plymouth, Minn.
Self expanding stents are commonly secured to a stent delivery system under radial compression or under axial tension in a collapsed state. Such a system can be comprised of an outer tubular member and an inner tubular member. The inner and outer tubular members are axially slideable relative to one another. The stent (in the collapsed state) is mounted on the stent delivery system surrounding the distal end of the inner tubular member. The outer tubular member (also called the outer sheath) surrounds the stent at the distal end.
Prior to advancing the stent delivery system through the body lumen, a guide wire is first passed through the body lumen to the deployment site. The inner tube of the delivery system is hollow throughout at least a portion of its length such that it can be advanced over the guide wire to the deployment site. The combined structure (i.e., stent mounted on stent delivery system) is passed through the patient's lumen until the distal end of the delivery system arrives at the deployment site within the body lumen. The deployment system and/or the stent may include radiopaque markers to permit a physician to visualize positioning of the stent under fluoroscopy prior to deployment. At the deployment site, the outer sheath is retracted to expose the stent. The exposed stent is free to self-expand within the body lumen. Following expansion of the stent, the inner tube is free to pass through the stent such that the delivery system can be removed through the body lumen leaving the stent in place at the deployment site.
Stent delivery systems may be comprised of an over the wire (OTW), rapid exchange (RX), or fixed wire (FW) delivery catheter. OTW delivery catheters allow a guidewire to pass through a lumen that extends over the entire length of the delivery catheter. RX delivery catheters allow a guidewire to pass through a lumen that extends over a partial length (usually 10-30 cm) of the delivery catheter. OTW delivery catheters provide better support than RX delivery systems yet they require the use of longer guidewires which can be cumbersome to handle. FW delivery systems are very simple in that they do not have a guidewire lumen. The FW system is advanced to a treatment site without the benefit of tracking over a pre-placed guidewire. To assure that a stent can be delivered to the intended treatment site stent delivery catheters with stents mounted thereon must have at least adequate trackability, flexibility, and kink resistance and stents, once implanted, must have at least adequate radial force, kink resistance, and fatigue life.
Stents implanted in some locations can require different physical attributes than stents implanted in other locations. Stents used in coronary or renal arteries can be fairly short because the stenosed region of the vessel is generally fairly short whereas stents used in the legs often must be very long so that disease typical of leg vessels can be treated without overlapping short stents.
In other examples, stents implanted in arteries must have high pulsatile fatigue life to withstand the small diameter changes caused by the artery diameter changes that occur with every heart beat whereas stents implanted in non-vascular conduits do not require this attribute. In a further example, stents implanted in limb vessels such as the superficial femoral artery (SFA) must have high fatigue life to withstand vessel length and orientation changes that occur when the leg is bent or straightened whereas stents implanted in coronary vessels need not meet this requirement. Long stents also must be forgiving of having an implanted length quite different (usually longer) from the design length as a result of physician technique during stent implantation and the practical limitations of stent delivery systems.
Unfortunately, a common problem with stents deployed in some vessels such as the SFA is that the stents fracture over time due to the dynamic loading environment of the vessel. Problems secondary to stent fracture can include intimal hyperplasia, pain, bleeding, vessel occlusion, vessel perforation, high in-stent restenosis rate, non-uniform drug delivery profile, non-uniform vessel coverage and other problems, and re-intervention may be required to resolve the problems. Stent fracture rates in the SFA range from 2% to 28% at 1-3 years after implantation, as reported in various clinical trials (Durability I, Resilient., Scirocco and Absolute) and research papers (for example Prevalence and clinical impact of stent fractures after femoropoliteal stenting; J. Am. Coll. Cardiol. 2005 Jan. 18; 45(2): 312) and Long-Segment SFA stenting—The Dark Sides: In-Stent Restenosis, Clinical Deterioration, and Stent Fractures; J Endovasc Ther 2005; 12:676-684.
While stents commonly achieve high pulsatile fatigue life, they often have insufficient resistance to fracture under in-patient loading conditions other than pulsatile, such as longitudinal extension, torsion, and flexion, as is appropriate for some implantation sites. Stents implanted in the popliteal artery, iliac artery, femoral artery, tibial artery, carotid, femoral-popliteal and other sites can suffer from large amounts of axial, flexural, or torsional cyclic loading. Recent data has indicated that some stents, such as those implanted in the SFA, must withstand axial elongation/compression of up to 35% as implanted. While attempts have been made to improve the fatigue resistance of implantable stents, an implant having suitable attributes (especially high fatigue life) for these demanding anatomical locations has yet to be developed.
What is needed is a stent that can be easily manufactured and that will survive without fracture when implanted in locations that experience high mechanical forces produced by patient activity in addition to those forces produced by the beating heart. Also needed is a stent which will have high fatigue life after implantation and a stent that will survive without fracture despite elongation during implantation.